Solvent based coating compositions are known which employ high molecular weight (e.g. 2,000 to 10,000) polymer resins having crosslinking functionality, and a suitable crosslinking agent. Typically, such coating compositions are applied to a substrate, for example, by spraying, and are then cured by baking the coated substrate at an elevated temperature suitable to drive off the organic solvent and to promote the crosslinking reaction. The resulting thermoset coating, if sufficiently humidity, solvent and corrosion resistant, can provide aesthetic and functional advantages including corrosion protection for the underlying substrate.
Coating compositions comprising such high molecular weight polymer resins typically comprise only 25% to 50% solids so as to be sprayable or otherwise conveniently applicable to a substrate. The viscosity of coating compositions of higher solids content is typically too high for this purpose. Conventional epoxy ester based automotive vehicle spray primers, for example, typically have a volatile organic content ("VOC") of approximately 540 g/l.
Elimination of the volatile organic solvent portion during curing of these conventional low-solids coating compositions presents toxicity and in some cases flammability hazards. Furthermore, bulk volume of these coating compositions is relatively large and therefore presents undesirable material handling difficulties, and added expense. Furthermore, excessive solvent losses and/or solvent recovery equipment add considerable expense to the coating operation. Recently, governmental regulations on hydocarbon emissions, particularly applicable to automotive coating operations, mandate a significant reduction in volatile organic content for coating compositions. Thus, for example, governmental guidelines for 1982 presently require that emissions of volatile organics from automotive vehicle primer coating operations be reduced to that equivalent to using coating compositions of no greater than 350 g/l (2.9. lb./gal.) VOC. To meet that guideline, coating compositions of VOC greater than 350 g/l can be employed in conjunction with emissions treatment equipment to achieve the specified emissions limit. Such treatment equipment presents significant additional expense, however, and thus there is a great need to provide coating compositions of VOC reduced near to, or preferably even lower than, the 350 g/l governmental limit.
In response to these concerns, high solids coating compositions have been suggested which, typically, employ low molecular weight multi-functional adducts or copolymers in combination with multi-functional crosslinking agents. These high solids coating compositions are less viscous and, therefore, can be applied by spraying, for example, with far lower VOC than was possible with conventional epoxy ester based coating compositions or other conventional coating compositions comprising high molecular weight polymer resins. After application to the substrate, high solids coating compositions are cured by baking at a cure temperature, that is, at an elevated temperature suitable to drive off the volatile organic content and to promote polymerization and crosslinking of the multi-functional low molecular weight component(s).
Typically, high solids coating compositions yield cured coatings having polymeric networks that differ significantly in structure and morphology from the polymeric networks provided by conventional, low solids coating compositions comprising high molecular weight polymers. Consequently, the physical properties of the coatings provided by such high solids coatings compositions can differ significantly from those of the cured coatings provided by the conventional, low solids coating compositions. In particular, the cured coatings obtained from known high solids coating compositions can be inferior in that they can be less flexible, less solvent resistant, less adherent to the substrate and/or for other reasons provide less corrosion inhibition for the underlying substrates. Accordingly, it would be highly desirable to provide a coating composition comprising low molecular weight materials suitable for use in high solids, solvent based coating compositions and yet which, upon curing, form coatings having physical properties, particularly corrosion resistance, comparable to or better than the physical properties of coatings obtained from conventional low solids solvent based coating compositions.
Accordingly, it is an object of the present invention to provide novel resin compositions suitable for use in high solids, solvent-based coating compositions. In this regard, it is a particular object of the invention to provide novel coating compositions which are curable by chain-extension and crosslinking during cure, in situ, on the surface of a substrate to form polymeric coatings similar in properties to those obtainable through use of conventional low solids, solvent-based coating compositions.
It is a particular object of the invention to provide a coating composition of sufficiently low VOC to facilitate compliance with governmental guidelines. It is also an object of the invention to provide such a coating composition which can be applied to a substrate by spraying or other known method.
It is another object of the invention to provide a method of making a coating on a substrate, which coating has advantageous physical properties including humidity, solvent and corrosion resistance. Additional aspects and advantages of the invention will be apparent from the following description thereof.